To understand nature is to describe it in proper methods. Here, we consider human or human activity is a part of nature. When we find out how things are occurring in natural or human world, it must be described in a suitable way. In most cases, we cannot describe whole nature exactly as it is, because nature looks too much complicated on the whole. Only we can do is that we firstly fix a view point and then describe nature as it looks from the view point. This work is carried out by presenting a model of nature. By modeling, we cut off many things which are not important in the context of...